This invention relates to an integrated optical device to be included in a transmitter for an optical fibre transmission system. In particular, this invention relates to a modulator with integrated attenuator to be included in a transmitter for a WDM optical fibre transmission system based on erbium doped fibre amplifiers, using pre-adjusting of the channel powers. Further, this invention relates to a method for reducing crosstalk between devices integrated on a substrate.
There is a considerable interest in using erbium doped fibre amplifiers (EDFA) to amplify weak optical signals for both local and trunk optical communications networks. The rare earth doped optical amplifying fibres are low in cost, exhibit low noise, provide relatively large bandwidth which is not polarization dependent, display substantially reduced crosstalk problems and present low insertion losses at the relevant operating wavelengths which are used in optical communications.
A particular area of current concern is that of increasing the capacity of an existing optical fibre communication system. Recent advances in erbium doped fibre amplifier technology suggests that increased capacity can be best obtained with wavelength division multiplexing (WDM) channels.
A major problem in implementing WDM channels on an optical transmission system is the absence of gain equalization. More specifically, because of non-uniform wavelength-dependent gain profile and saturation characteristics of optical fibre amplifiers, such as erbium doped fibre amplifiers, each channel in a WDM system will experience a different optical gain which, in turn, can result in an excessive bit error rate for channels with low gain.
A solution to this problem is indicated in U.S. Pat. No. 5,225,922 to ATandT Bell Laboratories, that discloses an optical transmission system in which the output powers and the signal-to-noise ratios of the channels of a WDM system are selectively equalized by adjusting the optical input signal powers. The power adjusters can be either optical amplifiers or optical attenuators or any device which can be used to selectively increase or decrease the power of the signal of each channel.
Integrated optics devices based on lithium niobate technology are well known in the field of WDM systems (see for example S. Bosso, Applications of lithium niobate integrated optic in telecommunication systems, Proc. SPIE Vol. 3620, p 34-37, Integrated Optics Devices IIIxe2x80x94The International Society for Optical Engineering, March/1999).
Optical modulators are among the most commonly used integrated optical components. They function by controlling the amount of light transmitted into a fibre optic link from a continuous wave (CW) laser, which emits polarized light. A commonly used optical modulator for digital applications consists of a Mach-Zehnder interferometric waveguide structure, having two Y-junctions and two waveguide arms between them, integrated on a lithium niobate substrate with travelling wave electrodes. Optimization efforts have been performed over the last years to reduce the driving voltage of digital Mach-Zehnder modulators and typical values of 3-4 V driving voltage (at 2.5 Gbit/s) are now reproducibly achieved on commercial devices. Recently, new devices have been introduced, integrating on the same lithium niobate substrate a variable attenuator with the modulator, in order to perform the adjusting of the power-per-channel for compensating EDFA gain shape.
An example is the xe2x80x9cOC192, Integrated 10 Gb/s Amplitude Modulator and Attenuatorxe2x80x9d, sold by SDL Integrated Optics, Model IOAP-MOD9189-F-F-O.
According to the data sheet by SDL Integrated Optics dated Sep. 24, 1999, this modulator has the benefit of a low drive voltage for ease of use with a wide range of commercially available drivers. With regards to the attenuation port, the same data sheet reports a value of driving voltage comprised between 8V and 12V.
Another example is the xe2x80x9c10 Gb/s Data Modulator with Integrated Variable Optical Attenuator (VOA)xe2x80x9d, sold by JDS Uniphase, Model 10150-002193.
According to the data sheet by JDS Uniphase dated November/1999, this modulator has a driving voltage for the VOA section not higher than 4V.
Y-branch electrooptical attenuators are known in the art. Generally speaking, these devices comprise a Y-branch waveguide structure with electrodes deposited near the waveguides. Typical voltages applied to the electrodes are higher than 15-20 V and can reach up to 30-50 V.
U.S. Pat. No. 5,970,201 to Lucent Technologies discloses a circuit for regulating optical power levels. Electrooptic Y-branched attenuators are used to control optical output intensity by means of feedback loops from the outputs of the Y-branch attenuators to the electrodes of the attenuators, which determine the amount of light going to the outputs
The problem faced by the above patent is to obtain a polarization independent attenuator which can be inserted in an optical network for controlling the power of optical signals having random variations in the state of polarization, as in optically amplified transmission systems when one or several wavelength channels are added or dropped. The use of a fast feedback loop together with a Y-branch attenuator allows to obtain a polarization independent automatic power controlling device, which can be used in combination with an optical switch array.
Applicant has realized a modulator with integrated attenuator by using a Y-branch attenuator integrated on the same substrate of a Mach-Zehnder modulator.
Applicant has found that the high driving voltage of a Y-branch attenuator is not a critical issue for the integration of the Y-branch attenuator on the same substrate of the Mach-Zehnder modulator.
Applicant has found that, in order to meet the requirements of a WDM system using pre-equalization of the channels, a critical issue for an electro-optical device comprising a Mach-Zehnder modulator and a Y-branch attenuator integrated on the same substrate is the reduction of crosstalk between the two integrated devices. Such crosstalk is caused by portions of unguided optical power spread on the substrate starting from waveguide sections belonging to the upstream device, which optical power is then re-guided by waveguide sections belonging to the downstream device.
In this respect, Applicant has found that particular arrangements of the Y-branch attenuator with respect to the Mach-Zehnder modulator can effectively reduce the crosstalk between the two devices, whereas other arrangements suffer from crosstalk up to values that are unacceptable for a WDM system.
In particular, Applicant has found that a configuration comprising a Mach-Zehnder modulator coupled to one of the arms of the Y-branch attenuator effectively reduces the crosstalk between the two integrated devices, with respect to a configuration comprising a Mach-Zehnder modulator coupled to the common waveguide of the Y-branch attenuator.
Applicant has also found that the use of a tilted Y-branch attenuator effectively reduces the crosstalk.
Applicant has also found that means for filtering and/or xe2x80x9cquick spreadingxe2x80x9d the portion of optical power radiated on the substrate between the two integrated devices can be effectively implemented for reducing the crosstalk.
In one aspect, the invention relates to an integrated optical device comprising:
a planar substrate,
a Mach-Zehnder modulator integrated on said substrate
a Y-branch optical attenuator integrated on said substrate to provide an attenuation range, said Y-branch attenuator being optically coupled to said Mach-Zehnder modulator, and
means for reducing a crosstalk between said Y-branch attenuator and said Mach-Zehnder modulator, whereby the extinction ratio of the optical device is at least 18 dB on an attenuation range of at least 6 dB.
Preferably, the extinction ratio of the optical device is at least 20 dB on the attenuation range.
In one embodiment, the Mach-Zehnder modulator is optically coupled to one arm of the Y-branch attenuator.
In an alternative embodiment, the Mach-Zehnder modulator is optically coupled to the common waveguide of the Y-branch attenuator
The means for reducing the crosstalk may comprise disposing one arm of said Y-branch attenuator parallel with respect to a longitudinal direction.
The means for reducing the crosstalk may comprise a filter of an unguided radiation spreading on said substrate, the filter being arranged in a region between said Y-branch attenuator and said Mach-Zehnder modulator.
Such a filter may comprise metal strips disposed on the side of a connecting optical waveguide coupling the Y-branch attenuator and the Mach-Zehnder modulator.
A gap between the metal strips can be about 170% of the MFD of said connecting optical waveguide.
In an example, the length of the metal strips is about 4 mm.
Advantageously, the metal strips are integrated in an extension of the electrodes of the Y-branch attenuator, so that the gap between the electrodes progressively increases from about 100% to about 170% of the MFD of the connecting waveguide in a first portion of the extension and is about 170% of the MFD of the connecting waveguide in a second portion of the extension.
The means for reducing the crosstalk may comprise waveguides for the Y-branch attenuator having a width lower between 5% and 16% with respect to the width of the waveguides of the Mach-Zehnder modulator.
Preferably, the width of the waveguides of the Y-branch attenuator is 8% less than the width of the waveguides of the Mach-Zehnder modulator.
In a second aspect, the invention relates to an integrated optical device comprising:
a substrate,
a Mach-Zehnder modulator integrated on said substrate
a Y-branch attenuator integrated on said substrate,
said Mach-Zehnder modulator being optically coupled to one arm of said Y-branch attenuator.
In a third aspect, the invention relates to a method for reducing the crosstalk between at least two devices including optical waveguides integrated on a substrate, each of said optical devices including at least one multimodal section of optical waveguide, the crosstalk being generated by unguided optical radiation propagating on said substrate in a region comprised between said optical devices, said method comprising filtering said unguided radiation in said region.
In a fourth aspect, the invention relates to a transmitting module comprising:
a laser source for emitting an optical signal,
an integrated optical device for modulating the intensity of said optical signal, comprising a Mach-Zehnder modulator formed on a substrate,
a Y-branch optical attenuator formed on the same substrate, optically coupled to said modulator, to provide an attenuation range, and
means for reducing a crosstalk between said Y-branch attenuator and said Mach-Zehnder modulator, whereby the extinction ratio of the optical device is at least 18 dB on an attenuation range of at least 6 dB
In a fifth aspect, the invention relates to a transmitting module comprising:
a laser source for emitting an optical signal,
an integrated optical device for modulating the intensity of said optical signal, comprising a Mach-Zehnder modulator formed on a substrate,
and a Y-branch optical attenuator formed on said substrate,
said Mach-Zehnder modulator being optically coupled to one arm of said Y-branch attenuator.
Preferably, the Y-branch attenuator is located upstream with respect to the Mach-Zehnder modulator.
In an alternative embodiment, the Y-branch attenuator is located downstream with respect to the Mach-Zehnder modulator.
The integrated optical device may comprise a dummy waveguide coupled to the second arm of said Y-branch attenuator and a feedback circuit optically connected to said dummy waveguide, said feedback circuit comprising electrical control circuits for controlling the wavelength of said emitted signal.